Apache HTTP Server mod_tcl set_var Format String Vulnerability

iDefense Security Advisory 10.13.06
http://www.idefense.com/intelligence/vulnerabilities/
Oct 13, 2006

I. BACKGROUND

The mod_tcl module for the Apache httpd v2.x is a scripting module that
allows a TCL developer to create server side script pages in TCL. It is
available as a contrib package for several distributions.  For more
information, see http://tcl.apache.org/

II. DESCRIPTION

Remote exploitation of a format string vulnerability in the mod_tcl
module for the Apache httpd v2.x could allow attackers to execute
arbitrary code in the context of the httpd. 

Due to programmer error, user supplied data is passed as the format
string specifier to several calls to an internally defined variable
argument function.  The function 'set_var' is declared as follows:

  mod_tcl.h:117:void set_var(Tcl_Interp *interp, char *var1,
                             char *var2, const char *fmt, ...);

Several insecure calls to this function are made through out the code,
as seen below:

  tcl_cmds.c:437:         set_var(interp, nm_var, (char*) key,
                                  (char*) val);
  tcl_cmds.c:2231:                set_var(interp, nm_env, env[i],
                                          sptr + 1);
  tcl_core.c:650:                         set_var(interp, namespc,
                                                  vl[i].var2,
                                                  vl[i].var3);

III. ANALYSIS

Successful exploitation allows remote attackers to gain local access to
the vulnerable system in the context of the affected httpd. To exploit
this vulnerability, the attacker must know the location of at least one
tcl server script that is configured to use this module for processing.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in version
1.0 of mod_tcl for Apache 2.x.

V. WORKAROUND

iDefense is currently unaware of any workarounds for this issue.

VI. VENDOR RESPONSE

The Apache mod_tcl team have addressed this vulnerability with mod_tcl
version 1.0.1.  It is available from http://tcl.apache.org/mod_tcl/

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-4154 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

08/16/2006  Initial vendor notification
10/11/2006  Initial vendor response
10/13/2006  Coordinated public disclosure

IX. CREDIT

This vulnerability was discovered by Sparfell.

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X. LEGAL NOTICES

Copyright © 2006 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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